Road-to-vehicle communication has been under investigation in an effort to prevent collision accidents of vehicles on a sudden encounter at an intersection. In a road-to-vehicle communication, information on conditions at an intersection is communicated between a roadside unit and an on-board equipment. Such a road-to-vehicle communication requires installation of roadside units, which means a great cost of time and money. In contrast to this, a vehicle-to-vehicle (inter-vehicular) communication, in which information is communicated between on-board equipments, has no need for installation of roadside units. In that case, current position information is detected in real time by GPS (Global Positioning System) or the like and the position information is exchanged between the on-board equipments. Thus it is determined on which of the roads leading to the intersection the driver's vehicle and the other vehicles are located (See Patent Document 1, for instance).